search in array javascript how many item that appears an odd number of times. code example

Example: check if number appears odd number of times in array javascript

function findOdd(A) {
    let counts = A.reduce((p, n) => (p[n] = ++p[n] || 1, p), {});
    return +Object.keys(counts).find(k => counts[k] % 2) || undefined;
}